Heat treatment of metals



JIan. 23, 1940.

J. A. COMSTOCK HEAT TREATMENT 0F METALS Original Filed Aug.v 50, 1938 QJ :25: y

Snventor ff Camloo (ttorneg Patented Jan. 23, 1940 PATENT OFFICE 2,188,275 HEAT TREATMENT OF METALS John A. Comstock,`Toledo, Ohio, assignor to Surface Combustion Corporation, Toledo, Ohio, a corporation of New York Original application August 30. 1938, Serial No.

Divided and this application Decembel' 22, 1938, Serial No. 247,201. Renewed N0- vember 8. 1939 4 Claims.

This application is a. division of my application led August 30, 1938, Serial No. 227,487.

The present invention has for its object to provide for the heat treatment of steel articles and molybdenum high speed tool steel in particular in a manner to avoid decarburization, scaling and hydrogen sorption thereof.

'I'he heat-treatment of molybdenum high,

speed steel presents difficulties that 'are not encountered in the heat-treatment of other high speed steels. Thus, whereas tungsten high speed steel can be clean hardened with practically no decarburization in an atmosphere resulting from the partial combustion of hydrocarbon gases, molybdenum high speed steel must be coated with a refractory material such as boric oxide, water glass or the like, as otherwise it will become decarburized. It is accepted that tungsten high speed steel-is not decarburized in such an atmosphere due to the formation of an impervious stable oxide lm over the surface of the article, this lm being .initially formed at low temperatures in the preheat furnace and remaining throughout the high heat furnace treatment. In the case of molybdenum high speed steel, however, such a protective oxide film cannot be maintained at heat-treating temperatures since molybdenum oxide is extremely volatile at heattreating temperatures as is readily observed -during forging, a bluish white fume resembling cigarette smoke coming oif.

The nearer such an atmosphere is regulated to a non-oxidizing constituency, i. e., by increasing gas-air ratio, the more pronounced is the decarburization of the work. Consequently such atmospheres are limited to the production of a minimum degree of oxidation consistent with maximum allowable decarburization on such steels as form self-protecting oxide films at the particular. heat-treating temperature.

In the course of my experiments leading upto the present invention, I heated specimens of molybdenum high speed in an atmosphere of ordinary charcoal gas but without -satisfactory results. I also made numerous tests using tank gases including nitrogen, hydrogen, ammonia, propane, etc. Mixtures of 10% hydrogen and 90% nitrogen, which were deoxidized by passing over iron filings at a temperature between 600- and '700 F. and dehydrated by activated alumina, decarburized molybdenum high speed steel at heat-treating temperatures. Tank hydrogen alone similarly deoxidized and dehydrated also caused decarburization and, furthermore, to a greater degree than in the case of the mixed gases. Tank nitrogendried andr deoxidized as above did not decarburize the steel but the specimens were found to be etched with a crystalline pattern resulting apparently from a slight oxidization on heating followed by vaporization of the resulting molybdenum oxide at high temperature. It became evident, therefore, that` an inert gas, such as commercial nitrogen, could not be used for this purpose because it cannot be obtained suiliciently free from oxidizing gas even if it could be maintained in such 'a condition during use and also that hydrogen was an undesirable reducing gas for mixing with the inert nitrogen. These observations wereV conrmed by the results obtained when using ammonia.

As a result'of my experiments, I have discovered and this forms the basis of the present invention that molybdenum high speed steel can be successfully heat-treateddn an` atmosphere containing CO, CO2, N2 and Hz provided that the H2 content'isrkept down to a very low limit, namely, to an amount not in excess of three onehundredths (.03) partial pressure in atmospheres. In general it may be said that for best results, the protective atmosphere should be substantially free from water vapor; that the relative proportions of carbon monoxide and carbon di,- oxide should be such as to be in equilibrium with the steel at the prevailing temperature; that the hydrogen content expressed in partial pressure in atmospheres should be less than three one hundredths (.03), and that the nitrogen content expressed in partial pressure in atmospheres should be less than eighty-five hundredths (.85).

It is known from the Work of others, and notably Takahashi, what the ratio of the square of the partial pressure in atmospheres of carbon monoxide divided by the partial pressure in atmospheres of carbon dioxide should be to be in equilibrium with steel of any particular carbon content at any particular temperature. Thus, for a steel at a temperature of 1500 F. and havin'g a carbon content of eight tenths per cent (-80%), the said ratio is in the neighborhood of 12 whereas for the same steel at a temperature of 2200 F., the ratio is in the neighborhood of 100.

This is only another way of saying that at the higher heat-treating temperatures it is necessary to have an atmosphere practically free from CO2 in order to avoid decarburization in the absence of scale. Tables containing this data are well known'and widely used in the art. By referring to such tables it is possible to ascertain very quickly what the relative proportions of `At this point it may be stated that molybdenum high speed steel contains about 9% molybdenum, 4% chromium, 1% vanadium and .78% carbon and that such steel is today recommended to be hardened at from 2150 to 2250 F., the usual temperature being 2200 F. Conse- 'quently at a temperature in the neighborhood of 2200* F., the relative proportions of CO and CO2 expressed in partial pressures in atmospheres should be such that the ratio of (CO)2 co2 is in the neighborhood of 100. At the lower preheating temperatures, the said ratio would, of course, be lower but not lower than 10 for a temperature of 1500 F., this being the temperature at which the steel is allowed to soak thoroughly before being raised to the high heat temperature.

A protective atmosphere as above described does not form an oxide film on the work nor does it depend on the protection of such a lm for the prevention of decarburization. Furthermore, the low hydrogen content precludes harmful hydrogen sorption, i. e., diffusion and absorption of hydrogen into the metal being treated. Moreover. I have found that such a furnace atmosphere does not place a time limit at which the work may be held at temperature as is the case when using ther atmospheres since, as I have found, the Work may be left for periods of several hours without decarburizing or scaling. Obviously this degree of protection may well be the basis of a new order of physical properties of heat-treated' steels.

In accordance with the present invention, the protective atmosphere consists ofthe reaction products obtained by passing an oxygen-containing gaslow in hydrogen or hydrogen compounds (this gas being hereinafter sometimes termed base gas) through a bed or column of externally heated carbonaceous material which has been so conditioned by heat-treatment as to be substantially devoid of hydrogen or hydrogencontaining compounds whereby the amount of hydrogen in said reaction products is substantially dependent on the amount of hydrogen or hydrogen compounds initially contained in said base gas, the temperature of said carbonaceous material and the rate of flow of the base gas being so controlled as to produce the required or desired relative proportions of CO and CO2 to insure that the ratio of (C0)2 co2 .or coke or other equivalent material. 'I'he choice of base gas will be determined largely by its relatively low content of hydrogen or hydrogen compounds and by its nitrogen content. Included in such base gases are air, dried ue gas, dried ammonia combustion products, and special producer gas. Various mixtures of these gases may,`

of course, be used to arrive at the desired nitrogen content in the final reaction products.

Iny the accompanying drawing forming partIk of this specification, I have shown two forms of apparatus that may be used in the practice of the present invention. Fig. 1 is a vertical section of a heat-treating furnace embodying a built-in gas generator designed to operate on what I call the counterflow principle. Fig. 2 is a vertical sectional view of the same kind of a heat-treating furnace but embodying a builtin gas generator designed to'operate on what I call the parallel flow principle.

Like reference numerals indicate like parts in the two views.

In both views, the heat-treating furnace is shown as of the mulie type. ID indicates the mul'lle, the same being provided at its front end with the usual door II through which the work to be heat-treated may be placed in and removed from the,mufile. The door is shown as provided with a glass-covered sight hole I2 to permit visual inspection of the work in the mufile. The furnace setting for the munie is generally indicated at I 3, the same comprising a furnace chamber I4 which surrounds the muille for uniformly heating the latter. As is customary in muflle furnaces, burners (not shown) re into the furnace chamber I4 and said burners are controlled by temperature control apparatus (not shown) for controlling theftemperature of the muiie I0.

I5 indicates a hopper or magazine adapted to contain the carbonaceous material C hereinbefore referred to. The top of the hopper will normally be kept closed by a cover I6. For reasons presently appearing the hopper is provided with a vent generally indicated at I'I, the same being shown as comprising a vpipe I8 provided with a' hand valve I9 for controlling the effective discharge area of the vent.

In the form of apparatus shown in Fig. 1, the hopper I5 delivers the carbonaceous material to a vertical retort positioned within the furnace chamber I4 to the end that it may be heated to the same, or substantially the same, temperature as the muilie I0. The retort is shown as long enough to extend downwardly out of the furnace chamber with its lower end normally closed by any suitable removable closure, such as gate valves 2I, to permit removal of ash. Leading into the lower end of the retort is a pipe 22 through which the base gas hereinbefore mentioned is delivered to the retort for upward .flow through the heated carbonaceous material.

Said pipe leads from a source of supply of such gas and is provided with a flow adjusting valve 23. A gas connection 25 is provided between the muille I0 'and the retort 20. For reasons presently appearing, said connection is a substantial distance upwardly from the discharge end of the gas pipe 22 and is also a substantial distance downwardly from the top of the furnace chamber I4. With respect to the muille, said gas connection 25 is shown' as at the upper rear corner thereof but inasmuch as it is desirable that the gas enter the mufe proper near the floor thereof, there is provided in the muille a depending baille 26 which serves to direct the gas downwardly.

In the form of apparatus shown in Fig. 2, the retort to which the hopper I5 delivers is indicated at 20', it being noted that this retort, like the retort 20 in Fig. 1, is within the furnace chamber wherein the muille is positioned as a consequence of which the retort will be heated to the same or substantially the same temperature as the muilie. The gas connection between the muille and the retort is indicated at 25. The pipe through which the base gas is supplied to the retort is indicated at 22', and it will be noted that this pipe extends downwardly into the retort from above and terminates a substantial distance above the gas connection 25'. Ash resulting from the combustion of the carbonaceous material may be removed from the lower end of the retort byway of the gas connection. However, it will be readily appreciated that the lower end of the retort might well be extended downwardly below the gas connection to form a receptacle from which ash may be withdrawn.

In Fig. l, the retort 20 may be considered as comprising two zones A and B. Zone A- is the gas making portion of the retort and Zone B is the place where the carbonaceous material is` conditioned before moving into Zone A. In Fig. 2, the zones which correspond to the zones A and B in Fig. 1 are indicated at A' and B' respectively.-

For temperatures above 1850 F. the retort in both forms of apparatus will be made of a heatconductingl refractory which preferably will be silicon carbide. For temperatures below 1850, it may be made of heat-resisting alloy.

To produce the protective gas with the gas generator shown in Fig. l, the procedure is substantially as follows. Having made sure that the hopper vent l1 is open, the valve 23 in lthe base gas supply pipe 22 is opened to such degree that considerably more base gas will always be supplied to retort 20 than is required to meet the y requirements of the muiiie Il) to the end that some of said gas shall always ow through zone B (and nally out through the hopper vent l1) in a direction opposed to the downcoming carbonaceous material C whereby to prevent the volatiles given ofi by the heated carbonaceousmaterial in zone B from contaminating the reaction 'products produced in zone A. The same procedure is followed with the gas generator shown in Fig. 2, it being understood that some of the base gas from the discharge end of the supply pipe'22' will iiow upwardly through zone B'. By thus continuously purging the zone QB or B) where the carbonaceous material is preheated, it willy be readily appreciated that the hydrogen content of the reaction products entering the muille by way of the gas connection 25 (25') may be readily controlled within the limits hereinbefore mentioned by utilizing a base gas whose content of hydrogen or hydrogen-containing compounds is known.

The relative proportions of CO and C02 in the products of reaction are a function of the temperature to which the carbonaceous material is heated and by maintaining the retort at the same or substantially the same temperature as o ary to employ two furnaces, one of which is usual- 1550 F. (the work being allowed to soak thoroughly at this temperature) and the super heat furnace at a temperature of about 2200 F. As an indication of the difference in gas composition of the., reaction products entering the muiile from the zone A (A') and those issuing from the hopper vent when the retort is maintained at the two temperatures mentioned and when the hase gas is low pressure atmospheric air, the following examples are given:

EXAMPLE No. 1.--Retort at 1550 F.

Exim-u No'. 2.-Retort at 22oo F.

The so-called Hopper gas", that is to say, the gas issuing from the vent I'I is entirely unsuited for use as a protective atmosphere for molybdenum high speed steel. The so-called Muiile Hopper gas Mufe gas Percent Percent 7. 4 2. 1 0. 0 0. 0 Balance Balance gas, however, that is to say, the gas which enters the mume Il from the zone A (A') of the retort will protect not only molybdenum high speed steel but also other high speed steels and,'in fact, steels generally against decarburization, scaling and hydrogen sorption. In this connection it may be statedv that furnace atmospheres produced asv herein described are applicable to many heating operations carried out on metals, including and especially. annealing, normalizing, fpreheating, hardening, tempering, brazing, melting, forging, etc.

The method herein described of making 'a metal-protecting gas of controlled hydrogen content regardless of the initial content of hydrogen or hydrogen compounds in the carbonaceous material (e. g., carbon, coke, charcoal) is an im portant feature of the invention.

What I claim is 1. A furnace for heat-treating metal articles comprising, in combination, a work-treating chambe a retort having a gas outlet in communication with said chamber and having an inlet for carbonaceous material separate from the gas outlet, means for delivering to said retort a'suillcient volume of a reactive gas to insure simultaneous outiiow of gas through said outlet ,and inlet, means for regulating the outflow of gas through said inlet, means for feeding carbonaceous material to said retort by way of said inlet, and means for maintaining said chamber and retort at substantially the same temperature.

2. Heat treating apparatus comprising in combination,l a heat treating chamber, a retort for carbonaceous material in gas connection with said chamber, means for maintaining said retort and chamber at substantially the same temperature, means for feeding carbonaceous material to the retort, means for expelling from the retort against the incoming carbonaceous material the volatile impurities given off by said material as it becomes heated whereby to prevent said impurities from flowing to said chamber by way of said gas connection, and means for delivering to the retort at a point where said material is substantially devoid of said impurities an oxygen containing gas whereby to cause a ilow of gaseous reaction products to said chamber by way of said gas connection. v

3. Heat treating apparatus comprising in combination, a heat treating chamber, a retort for carbonaceous material in gas .connection with said chamber, means for maintaining said chamber and a retort at substantially the same temperature, means comprising a container for ieeding carbonaceous material to the retort at a point remote from said gas connection, means comprising said container for expelling from the retort the volatile impurities given off by the carbonaceous material as it becomes heated whereby to prevent said impurities from owing into said chamber by way of said gas connection, and means for delivering to the retort at a point where said material is devoid of said impurities a gas that will react with the carbonaceous material to produce carbonmonoxide.

4. A heat treating furnace comprising in combination, a retort for carbonaceous material, means for feeding said material to the retort in a descending column, a heat treating chamber in .gas connection with the retort, a furnace chamber wherein said retort and chamber are arranged, means for drawing o gas from the retort against the incoming carbonaceous material to draw 0H volatile impurities given oi by said material as it becomes heated to prevent said impurities from ilowing to said chamber by way of said gas connection, and means for delivering to said retort at a point where said material is devoid of said impurities a gas that will react with said material to produce carbon monoxide.

JOHN A. COMSTOCK. 

